1. Field of the Invention
The present invention relates to an SAW(Surface Acoustic Wave) filter package, and more particularly, to an SAW filter package which can reduce an electromagnetic feed-through for improving an SAW filter performance.
2. Background of the Related Art
Being a filter of a piezoelectric material, the SAW filter is widely used as an intermediate frequency filter because the SAW filter permits multipoint drive and reception, has excellent earthquake and impact resistances as being supported from a substrate, and permits independent control of amplitude and phase, and has an excellent filter performance.
Referring to FIG. 1, a related art SAW filter is provided with an input terminal IDT (Inter-Digital Transducer) 12 for providing an input signal Vs with an impedance Rs, an output terminal IDT 13 connected to an output impedance R.sub.0, and absorber 14 for absorbing a reflected wave.
Shown in FIG. 2 are details of the input terminal IDT 12 and the output terminal IDT 13, both are of a withdrawal weighting type, respectively having one side with an input port 15 and the other side with a ground terminal 17, and one side with a ground terminal 17 and the other side with an output port 16. And, there is a slightly tilted blocking bar 18 disposed between the input terminal IDT 12 and the output terminal IDT 13 having both ends connected to the ground terminal 17 for reducing the electromagnetic feed-through.
FIGS. 3A.about.3D illustrate details of a general SMD(Surface Mounted Device) package for mounting the SAW filter, wherein FIG. 3A illustrates a back of the package, FIG. 3B illustrates a side of the package, FIG. 3C illustrates a plan view of the package with a cover closed, and FIG. 3D illustrates a plan view of the package with a cover opened.
Referring to FIGS. 3A.about.3D, an SMD package is provided with a base 20 of ceramic, a metal plated frame 21 projected vertical from the base 20 along a periphery of the base 20, a plurality of gold plated pads 22 formed in sides of the base 20, a ground terminal 23 of a gold plating formed on an inside surface of the base 20, and a cover 24 for sealing the mounted filter. And, the SAW filter is mounted on the ground terminal 23 of the SMD package. The reference numeral 25 is a pad connection terminal.
Upon application of a high frequency electric field to the input terminal IDT 12, a surface acoustic wave is excited by a piezoelectric action in the related art SAW filter, transmitted to the output terminal IDT 13 through a surface of the piezoelectric material 11, and converted into the high frequency electric field again by a reverse piezoelectric action, to generate an electric signal. When the surface acoustic wave, generated upon application of a high frequency electric field to the input terminal IDT 12, is transmitted through surface of the piezoelectric material 11, there is an electromagnetic feed-through directly transmitted from the input terminal IDT 12 to the output terminal IDT 13 in a form of electric or emissive energy. This is caused by the close proximity of the input terminal IDT 12 and the output terminal IDT 13, which is increased as the applied frequency is increased and makes interference with the surface acoustic wave to deteriorate an SAW filter performance because the electromagnetic feed-through is transmitted at a speed of light different from an energy carried by the surface acoustic wave that is the basic operation principle of the SAW filter.
The electromagnetic feed-through is caused by the following major two reasons.
The first one is that, as in general upper and lower surfaces of the package are formed of conductive material and the piezoelectric material of the SAW filter is a ferroelectric material, the package with the SAW filter mounted thereon forms a dielectric waveguide because a ferroelectric material present between the input/output terminal IDTs, which are formed on surfaces of the piezoelectric material, and the package. In this instance, even though a wave fed through a dielectric waveguide is leaky, an energy transmitted by the electromagnetic feedthrough is relatively great in comparison to other secondary effect because the input terminal IDT and the output terminal IDT are at a close proximity.
Another one is that emission occurs between the input/output terminal IDTs and bonding wires. Since the input/output terminal IDTs and the bonding wires actually act as antennas, they emits electromagnetic waves within the SAW filter package, causing the electromagnetic feed-through. As shown in FIG. 2, in the related art, in order to reduce the electromagnetic feed-through, the metallic conductive blocking bar 18 is placed on the surface of the piezoelectric material 11 between the input terminal IDT 12 and the output terminal IDT 13, with the bar 18 tilted at a given angle and both ends of the bar 18 grounded. This configuration can reduce the electromagnetic feed-through because the electromagnetic feed-through from the input terminal IDT 12 to the output terminal IDT 13 through the surface of the piezoelectric material 11 is absorbed by the blocking bar 18 and discharged through the ground terminals at both ends thereof.
However, as there is only the blocking bar provided in the related art SAW filter package, the related art SAW filter package can not but remove a portion of the electromagnetic feed-through flowing through the surface of the piezoelectric material, allowing the other portion to cause interference with the surface acoustic wave, that deteriorates the filter performance.